1. Field of the Invention
The present invention relates to a technique applicable to an image reading apparatus that may generate image unevenness.
2. Description of the Related Art
There have been made available conventional image reading apparatuses equipped with an automatic document feeder (ADF) that can automatically feed documents one by one. The image reading apparatus includes an endless conveyance belt that can press a document onto a document positioning glass and also shift the document along the glass.
In general, the endless conveyance belt has inclined parts at the front and rear ends of its lower side (i.e., bottom) to smoothly guide a document to or from the document positioning glass. In other words, the conveyance belt floats against the document positioning glass at its front and rear ends. Thus, the conveyance belt cannot firmly press both a leading edge and a trailing edge of a document in a sub scanning direction, although the conveyance belt can firmly press the central region of the document.
The floating of the conveyance belt induces undesirable phenomenon, such as “image unevenness,” resulting from local deterioration in the document reading level changing depending on the sub scanning position. More specifically, the image reading level becomes lower (darker) at the leading edge and the trailing edge of the image, compared to the reading level at the central region of the image. Such a local deterioration in the image reading level is the main cause that generates image unevenness.
To solve this problem, Japanese Patent Application Laid-Open No. 2003-338908 discusses a technique capable of correcting image unevenness caused in the sub scanning direction due to the floating of a conveyance belt. However, the above-described conventional technique is only effective to eliminate the image unevenness in the sub scanning direction, and accordingly cannot correct image unevenness which may be caused when the conveyance belt deflects in the main scanning direction.
In this case, conveyance rollers driving the conveyance belt as well as driven rollers driven by the conveyance belt may enhance the image unevenness caused by the conveyance belt deflecting in the main scanning direction.